This skepticism should REALLY be directed at advertisements. In our hobby, many vendors depend on hobbyist ignorance to sell their products. Ignorance, fortunately, is a condition cured by knowledge. For example, when you see a statement like "replace what gets used up," you should ask yourself the following question, "If substance "X" gets used up, what happens to it and where does it go?" The most fundamental natural law is the law of conservation of matter and energy. Matter cannot be created or destroyed, nor can one element be converted into another. So, if you see a statement that some element is "used up" in aquaria, just what does that statement mean? Ask yourself, "How has it been used by organisms?" "Has it been exported from the aquarium?" If it hasn't been exported from the aquarium, it is still there, and it hasn't been "used up;" rather it simply has been moved from one place or form to another. Then you need to ask yourself, where is the documentary evidence that any specific material is used in any manner?